The invention is directed to a molded body made of graphite having nickel sulfide as a binding agent for the safe longtime embedding of radioactive and toxic wastes, and a process for the production of such a molded body.
Spent fuel elements from nuclear reactors after a certain period of time of intermediate storage must be sent to a final disposal. Worldwide for this purpose, there have been tried two methods, namely the reprocessing of the fuel element with return of the fuel material for the preparation of the fuel elements as well as separation and final storage of the fission products (highly active waste) and alternatively the direct final storage of the spent elements. In each case, there is formed highly active waste which must be stored in suitable geological formations for 1000 years and longer. Also, other highly radioactive and highly toxic waste must be safely kept away from the biosphere.
There have been proposed numerous types of containers for the safe longtime confinement of such radioactive and toxic waste which fulfill well the necessary conditions such as tight confinement at the pressures and temperatures which occur or resistance to corrosion by salt liquors. As container material, there have been used numerous metallic and non-metallic materials.
Since graphite has an outstanding resistance to corrosion, it has been proposed (German OS No. 2942092) to provide containers with a corrosion protective layer made of graphite. Since graphite molded bodies which have the necessary dimensions for receiving a fuel element are produced in neither gas nor liquid tight condition, there is provided a connected coating of pyrolytic carbon or silicon carbide. After the filling in of the fuel element the coated container should be closed with a similarly coated gas and liquid tight cover. Thereby, graphite gaskets or suitable adhesive agents should be employed. A substantial disadvantage of this container concept is the extraordinarily high industrial expense which is necessary for the production and coating of containers having large dimensions. Besides such large molded bodies cannot be coated within the necessary quality specifications.
It is also known to produce molded bodies made of a carbon matrix for embedding radioactive and toxic wastes by pressing graphite powder with a binding agent. As binding agent, there is preferably used nickel sulfide (German OS No. 2917437 and related Hrovat U.S. application Ser. No. 143,941, filed Apr. 25, 1980, now U.S. Pat. No. 4,407,742 the entire disclosure of which is hereby incorporated by reference and relied upon). Such molded bodies are very dense, and possess a good corrosion and leach resistance, especially against salt solutions. However, in many cases the thus produced molded bodies do not have the optimum resistance to corrosion and salt solutions. In the Hrovat U.S. application (and German OS No. 2917437 ) Example 2, which is the only one employing sulfur and nickel powder and forming nickel sulfide there is used 20 parts of sulfur and 36.7 parts of nickel metal powder by weight (corresponning to 54.5 parts of sulfur to 100 parts of nickel). The properties of the molded article of Example 2 were:
Density of the binding matrix: 3.1 g/cm.sup.3 PA1 Compressive strength: 73.8 MN/m.sup.2 PA1 Heat conductivity: 0.28 W/cm..degree.K PA1 linear coefficient of thermal expansion: 17.7 gm/m..degree.K
Therefore it was the problem of the present invention or provide a molded body made of graphite having nickel sulfide as binding agent for the safe longtime embedding of radioactive and toxic wastes which exhibits the highest possible resistance to corrosion and leaching out.